For existing diffraction gratings or prisms, dispersion of light output by the existing diffraction gratings or prisms is linear with respect to the wavelength, which means that the output angles of light output from the existing diffraction gratings or prisms are linear with respect to the wavelength of the incident light, since the output angles of light output from the existing diffraction gratings or prisms are dependent from the wavelengths of incident light. Therefore, spectroscopy made of traditional diffraction grating and prism can only be linear in wavelength, where the output angles of output light are nonlinear with respect to the frequency of the incident light,
In the field of optical coherent imaging, such as Optical Coherence Tomography (OCT), the general practice is to get the image by Fourier transform of acquired coherent spectrum. Since the acquired coherent spectrum is linear with respect to wavelength, it must be transformed from the wavelength domain to the frequency domain by the method of interpolation prior to the Fourier transform. Accordingly, the imaging speed is reduced. In addition, due to the non-linear effects in the frequency domain, the density of optical images is lighter and the noise of imaging is greater.
The traditional spectrometers are only applicable to light with certain wavelengths, if the wavelength of incident light is changed, it is required to change to a corresponding spectrometer, thereby limiting applications of the spectrometer.
Therefore, there exist needs in many situations for optical devices in which the dispersion of light is linear with respect to the frequency of incident light.